For several years we have been treating deep vein thrombosis with recombinant tissue plasminogen activator that is injected directly into the clots. We have now collected blood samples before and at several time points following the treatment to determine how long the drug has a systemic effect. We found that it only circulates in an active form for 1 - 2 hours after injection and then becomes inhibited by a rise in plasminogen activator inhibitor-1.